Several methods are known to extract albumin from vegetable raw materials, an alkaline extraction of the albumin at a pH of between 7 and 9 and at a temperature of 40 to 50° C. being the basis of these methods. The extracted albumin is then purified by centrifugation and acidified with a mineral acid until the isoelectric point of the albumin is reached. The albumin precipitated at the isoelectric point is again concentrated and purified by means of centrifugation. In this way, yields of 70% as related to the albumin contained in the raw materials are possible. However, it is a serious drawback that only such raw materials may be used in which the albumin has a low level of denaturation. Especially in exploring the most important vegetable albumin isolate, i.e., soya albumin isolate, the limitation to so called “white flakes” as a raw material is an economic drawback. White flakes are only obtained by means of a special drying method for the remainders of soya oil production. Normally, the remainders are dried less carefully, and they are then present in a form of so called “toasted flakes” in which the albumin is strongly denatured.
From DE 12 03 588 it is known to enhance the alkaline extraction process in exploring albumins from an albumin containing substance by means of a pre-treatment of an aqueous suspension of the substance with hydrogen peroxide within the alkaline range and with proteolytic enzymes. To this end, the pH of the suspension of the albumin containing substance is at first raised, then hydrogen peroxide is added and the temperature is raised to initiate peroxidation. Then, the pH is adjusted to 4 to 9, and enzymes are added to the suspension, the activity minimum of which is within this pH-range. Afterwards, the suspension is stirred for two hours. Preferably, vegetable enzymes like bromelaine, ficine and papaine are used. After the enzymatic hydrolysis, the albumin is dissolved in that the pH is raised to 9 to 12. At the same time, high temperatures are applied from which it is known that they result in albumin damages particularly in combination with the high pH. The multiple change of the pH in this known method requires great efforts in large scale applications and results in a high consumption of alkali and acid followed by a strong formation of salt.
It is also known from DE 44 29 787 C2 to use proteolytic enzymes for enhancing the solubility of albumins. Here, the starting material is at first extracted with alcohol, only afterwards it is mechanically broken up and then further enzymatically broken up in the acid to neutral range. The extraction of attendant material to the albumin is effected by means of a countercurrent process. A treatment with hydrogen peroxide can take place after the extraction of the albumin in this known method, too. Due to the use of alcohol for the extraction, the method requires high efforts which are disadvantageous. The explosion danger of the suspension and the inactivation of the used enzymes by the presence of alcohol have to be permanently considered. In the known method, the proteolyses is per definition limited to a pH-range of <9.5, only the pH-range of <7.5 being concretely described in DE 44 29 787 C2. On the other hand it is known to those skilled in the art that the solubility of albumin is particularly great, if the pH is over 9. All in all, the efficiency of the known method as compared to the cost to be spent for its application is only low so that the prepared albumin isolates are not competitive.
A method according the preamble of claim 1 is known from DE 43 39 743 C1. This method does without the use of alcohol. For dissolving the protein, however, a pH of over 11.5, particularly of about 12.5, is needed which is associated with a high consumption of alkali and of acid for a later neutralization. Prior to the proteolysis, the albumin containing substance is treated with a protease at a pH>10.5. The pre-treatment requires a comparatively large amount of enzyme, which results in a drawback with regard to the production cost. The comparatively high salt content at the end of the albumin, extraction of this known method, which in praxis requires an additional washing step, is a further drawback.
A method for extraction of albumin from an albumin containing substance which operates with high amounts of enzyme is also know from JP 2 076 597 A. An alkaline protease is added to an alkaline suspension of the albumin containing substance, the pH of which is between 10 and 13, and incubated at a temperature of 30 to 50° C. for 1 to 20 hours. Afterwards, neutralization and filtration of the suspension takes place. These indeed dramatic conditions of the albumin extraction result in extended albumin damages. As a result, it is only possible to obtain hydrolysates for cosmetic applications by the known method. It is unsuitable, however, for the preparation of albumin isolates as food.